Consumption of low crude protein (CP) diets causes elevation in fat accumulation in chickens, and this effect is independent of dietary essential amino acid levels. Thyroid hormones, because of their metabolic regulatory characteristics, might be an effective factor in lipogenesis. Therefore, a study was conducted to investigate the influence of low CP diets on hormonal function of the thyroid. Two hundred and sixteen male broiler chicks, 10 - 25 days of age, were fed experimental diets that contained 230, 210, 190 and 170 g CP/kg with adequate amounts of total and digestible essential amino acids at or above NRC (1994) recommendations. Two other diets were the same as the 190 and 170 g CP/kg diets but contained additional glycine and glutamic acid. Reduction in CP below 190 g/kg led to a decline in body weight and feed intake and an increase in fat deposition in body, as would be expected. Plasma T4 concentration decreased significantly in the birds on the 170 g CP/kg diet and supplementation of glycine and glutamic acid had no effect on hormonal levels of the thyroid despite the reduction in whole-body and abdominal cavity fat deposition. Plasma T3 concentration was not affected. Therefore, it is concluded that a reduction in circulatory levels of T4 that occurs in broiler chicks fed diets containing below 190 g CP/kg though with adequate essential amino acids, may be an effective method of increasing fat deposition.